1. Tsinghua‐Berkeley Shenzhen Institute (TBSI) Tsinghua University Shenzhen 518055 China
2. Institute of Materials Research Tsinghua Shenzhen International Graduate School (SIGS) Tsinghua University Shenzhen 518055 China
3. Shenzhen Geim Graphene Center Tsinghua Shenzhen International Graduate School (SIGS) Tsinghua University Shenzhen 518055 China
4. Department of Materials Science and Engineering University of California, Berkeley Berkeley CA 94720 USA